A process for treating molten slags from the non-ferrous metallurgy is already known by German Offenlegungsschrift nr. 2,727,618. In this process, a solid carbonaceous reducing agent is distributed onto the surface of molten slag where it forms a layer. A non-oxidizing gas is injected in an amount and with a flow rate so that molten slag is circulated, projected through the layer of carbonaceous reducing agent, and filtered by it. According to this process, the flow of non-oxidizing gas, which is injected into the liquid slag, is preferably between about 30 to about 100 Nm3/h per metric ton of slag (the unit Nm.sup.3 /h represents standard cubic meters per hour).
One drawback of this process is that the high flow of gas injected into the liquid slag creates agitation which is not compatible with the operation of an electric furnace with submerged electrodes. Normal operation involves a smooth, continuous operation which preferably extends over long periods without interruptions. For example, comparative to operation without injection of gas, this operation is electrically less stable, which is shown by sudden and violet fluctuations of the current and of the instantaneous power consumed by the furnace. Also, the power factor (cos .phi.) reaches values which are much lower than those prevailing with an operation without such injection. Consequently, special devices must be provided in order to correct the electrical fluctuations or penalties will eventually be applied by the power distributors. On the other hand, violent agitation of the slag will tend to accelerate wear on the furnace lining and to project liquid material on the roof and the side-walls of the furnace. These are factors which contribute to a reduction of the life of the furnace and a limitation of the possibilities of long lasting continuous operation.
Another drawback of this process is that high gas flow may lead to an important volatilization of metals whose vapour pressure is high at operating temperature. This may be considered an advantage for some metals, such as zinc, which are commonly recovered in the dust obtained from the gaseous phase during reduction smelting operations. This volatilization is however undesirable and must preferably be limited to a minimum for other metals, such as lead, which are preferably recovered directly in the liquid state from the slag to be reduced. If lead and zinc are simultaneously present in the slag to be reduced, a compromise has to be found between the volatilizations of both metals. In this situation, it is observed that an optimum often corresponds to relatively low volatilization rates, which are impossible to achieve under the conditions of the prior art process.
Another drawback of this process is that the high flow of gas blown into the slag leaves the bath at a high temperature. The gas absorbs thermal energy in significant amounts, which can be difficult to recover efficiently and which has to be compensated by addition of electric energy to the electrodes of the furnace.
A further drawback of the prior art process is the large consumption of non-oxidizing gas. Consequently, the cost of the gas may constitute an important fraction of operating costs and make the operation uneconomical, for example, when untreated slag contains relatively small amounts of valuable metals or when the value of these metals is relatively low. The aim of the present invention is to avoid these various drawbacks.